The present invention is directed to a method for synchronizing a clock means of a telecommunication switching system to which is supplied at least at times, reference clock signals from a plurality of external reference clock sources.
In synchronous digital telecommunication networks, central clock means of individual digital exchanges are hierarchically synchronized according to the master/slave principle. A single, frequency-defining clock generator (master clock) is thereby provided for the entire network and all other clock generators are synchronized to its clock frequency. Particularly for larger networks, however, this method has the risk that a fault in the central clock generator will cause the entire network to fail. In order to reduce the extent of the failure range, redundant clock supply lines are provided in this method and relatively exact, local clock generators are also utilized so that operation can be maintained in case of a malfunction at least with what is still an acceptable slippage rate (Bergmann, Lehrbuch der Fernmeldetechnik, Vol. 1, 5th Edition, Berlin 1986, page 485).
European Patent Application 90105379.3 discloses a method for synchronizing a clock means of a clocked electrical system. During normal operation, the telecommunication exchange is synchronized by an external reference frequency and, given a long outage of the external reference frequency, a normal internal frequency for the system is activated which is spatially separated from the clock means. When more than one external reference frequency is available, the normal internal frequency for the system is only activated when all external reference frequencies have failed. Given outage of a currently utilized external reference frequency, a switch to a different external reference frequency is undertaken. How this switching occurs when a plurality of further, external reference frequencies are available given outage of the currently connected external reference frequency thereby remains unanswered.